Ultrahigh Strength Steel: Development of Mechanical Properties Through Controlled Cooling

Structural steels with very high strength are referred as ultrahigh strength steels. The
designation of ultrahigh strength is arbitrary, because there is no universally accepted
strength level for this class of steels. As structural steels with greater and greater strength
were developed, the strength range has been gradually modified. Commercial structural
steel possessing a minimum yield strength of 1380 MPa (200 ksi) are accepted as ultrahigh
strength steel (Philip, 1990). It has many applications such as in pipelines, cars, pressure
vessels, ships, offshore platforms, aircraft undercarriages, defence sector and rocket motor
casings. The class ultrahigh strength structural steels are quite broad and include several
distinctly different families of steels such as (a) medium carbon low alloy steels, (b) medium
alloy air hardening steel, (c) high alloy hardenable steels, and (d) 18Ni maraging steel. In the
recent past, developmental efforts have been aimed mostly at increasing the ductility and
toughness by improving the melting and the processing techniques. Steels with fewer and
smaller non-metallic inclusions are produced by use of selected advanced processing
techniques such as vacuum deoxidation, vacuum degassing, vacuum induction melting,
vacuum arc remelting (VAR) and electroslag remelting (ESR). These techniques yield (a) less
variation of properties from heat to heat, (b) greater ductility and toughness especially in the
transverse direction, and (c) greater reliability in service (Philip, 1978). The strength can be
further increased by thermomechanical treatment with controlled cooling.